Conventional cover or intermediate layer materials for golf balls include ionomer resins and polyurethanes. Chemically, ionomer resins are a copolymer of an olefin and an ethylenically unsaturated α,β-carboxylic acid having 10 to 100 percent of the carboxylic acid groups neutralized by a metal ion. Polyurethanes are typically formed from a prepolymer of polyol with isocyanate and a curing agent. Ionomer covers are virtually cut-proof, but they provide inferior spin and feel to the golf balls.
Polyurethanes are typically formed from polyols, isocyanates, and curing agents. These compounds can be mixed together in the one-shot method to form the polyurethanes. A preferred alternative method involves a two-stage process, in which the polyol is first reacted with the isocyanate to form a polyurethane prepolymer, and then the prepolymer is reacted with the curing agent to afford the polyurethane. Conventional polyurethane covers generally have excellent feel and good durability, but are inferior in resilience when compared to ionomer covers. The employment of aromatic compounds in conventional polyurethane covers typically results in their lack of weatherability in general and light stability in particular. The color instability caused by both thermo-oxidative degradation and UV-induced photodegradation typically results in “yellowing” or “browning” of the polyurethane cover, an undesirable characteristic for golf ball covers, which are generally white.
Polyacrylate polyols are capable of providing improved lightfastness in polyurethane compositions, and are suitable for use in golf balls. Conventional polyacrylate polyols are copolymers comprising hydroxyl-group-containing acrylic monomers. Typically, the acrylic monomer is a hydroxyalkyl acrylate or hydroxyalkyl methacrylate, wherein the hydroxyalkyl group is hydroxyethyl, hydroxypropyl, hydroxybutyl, and the likes thereof. One limitation of these conventional polyacrylate polyols is the random distribution of the hydroxyl groups throughout the backbone of the polymer, such that they can not be used for the synthesis of linear polyurethanes. The high level of hydroxyl functionality of these polyacrylate polyols may result in premature gelling when they are reacted with isocyanates. Polyurethanes using such polyacrylate polyols have high crosslinking density, high hardness, high glass transition temperature, and poor flexibility, which limit their application in golf balls and other sport equipment.
Therefore, a continuing need remains for golf ball materials and compositions with superior resistance to cut, scratch, abrasion, discoloration, moisture, and other wearing and weathering elements that provide improved performance characteristics such as resilience and feel to the golf ball. It would be particularly desirable to incorporate polyacrylate compounds having terminal isocyanate-reactive functional groups and low active hydrogen functionality.